Amino acids are like building blocks of human body. They are connected in a manner that is dictated by the DNA and they form proteins, which are responsible for most of the processes that happen in human bodies. One of the defining characteristics of living organisms is the ability of its smallest molecular structures to self-assemble with great precision. The folding of the proteins in human bodies are one of the most fundamental and universal examples of such self-assembly processes; nonetheless, the mechanisms by which these processes work is still under investigation by scientists in multiple fields [1].
Cells have certain quality control mechanisms that ensure that the proteins are folded correctly because failure to do so gives rise to malfunctions of normal processes and hence to disease. Among all the possible structures that can be formed by the polypeptide chains, amyloid fibril aggregates are considered to be of one of the most biologically important protein structures due to their implication in numerous neurodegenerative diseases [2]. The diseases that have been connected to amyloid fibril formation include Alzheimer’s disease [3], Parkinson’s disease, and prion disease [4]. As a result, understand how and why aggregation of peptide chains happens is crucial in the fight against these diseases.
Fibrillization
Based on the conditions and the environment, protein can take on multiple conformations. Figure 1 shows some of the possible structures that can be formed from the polypeptide chains.
Figure 1. A unified view of some of the types of structures that can be formed by the polypeptide chains [2].
The fibrillization of peptides, as mentioned earlier, is relevant to many diseases and is based on the deposition of amyloids. The term amyloid refers to protein deposits that adopt fibrils formation based on the cross-β sheet structure [6]. Structuring through β-sheets is common in many protein aggregates. During protein fibril formation, β-sheets must aggregate in a rigid fashion to create the specific structure that is characteristic of the amyloids [7].
One of the major proposed theories in development of amyloid diseases is the amyloid cascade hypothesis, which states that malfunction of proteins leads to the production of amyloid proteins and eventually aggregation into cell-disrupting amyloid plaque [2]. In addition, it was verified that amyloid fibrils could be transmitted to other sites to promote further fibril formation. At this point the disease becomes infectious [7]. This fact makes it crucial to figure out how to control the formation of fibrils.
Many experiments that were done during research of the fibril formation show that there are certain factors that either promote or inhibit the formation of the fibrils. Over time it was been verified that the following factors are important in the formation of amyloid fibrils: concentration, ionic strength, pH level, solid substrates, temperature [7]. Although more research is being conducted to figure out how exactly these factors affect the onset of aggregation, they all have significant role in assembly of the fibrils.
While it has been verified that in order for aggregation to start and amyloid fibrils to form, certain conditions mush be met; once amyloid fibrils do form, they are very thermodynamically stable and the reverse process does not occur spontaneously [8]. Therefore, scientists are trying to find strategies to either inhibit aggregation altogether and prevent the amyloid fibrils from forming or to change the way aggregation occurs.
Factors that Affect Aggregation
Fortunately, the topic of aggregation of peptides had been fairly popular and a lot of progress is being made in discovering how to control the outcomes of the processes. More research became available because many scientists started using short peptides as models for amyloid formation. Since most proteins have long polypeptide chains, it makes them a little more complicated to work with. Moreover, the synthesis of the large peptides is expensive and difficult. As a result, much current work is done on short peptide sequences [9].
Since work with shorter peptide sequences allowed for more detailed research, there are several, both physiological and nonphysiological, conditions that have been investigated to break, disrupt, and destabilize amyloid fibrils. First of all, actual physical energy has been applied to break down fibril aggregates. There have been experiments conducted where ultrasonification treatments of the aggregated peptides resulted in shorted amyloid fibril fragments. In addition to breaking the fibrils into smaller fragments, ultrasonification is a powerful technique, which allows for producing monodispersed fibrils with a well-defined molecular size [10]. Also, the opposite of this concept is useful in cases where undisturbed aggregation is used to produce elongated aggregates during the lag phase, or the time before aggregation is complete [11].
Another factor that affects the process of aggregation is high temperature. It has been found during one study that temperatures above 100 °C happen to disrupt the strength of hydrogen-bond networks [12] that are crucial for the rigid amyloid fibril structure [13]. In the observed experiments no amyloid formation was observed at and over 140 °C. In addition, the morphology of actual fibrils changed with increasing temperature. It was found that as the temperature increases, the size of the fibrils decreases and their shape is not as linear as the shape of the fibrils at lower temperatures [12].
Additionally, charge is very important factor in the process of aggregation. Charge is crucial for the structure stability; therefore drastic changes in the pH level or the level of salt in the environment can lead to fibril destruction [8]. Through changing the charge of the amino acids along the peptide, the pH value determines intramolecular protein interactions and electrostatic intermolecular interactions between proteins. As pH level decrease, the protein becomes more acidic and the non-selective aggregation is favored. Experimentally this results in the formation of stiff fibril morphology, although β-sheet alignment may be found in aggregates at all pH levels [7].
Factors like protein concentration and solvent quality also affect aggregation. Both of them have strong influence on what type of aggregates will form, for example fibrils or amorphous assemblies. In combination with the temperature, these two factors also affect how fast the aggregation is going to proceed. In higher temperature there is clear correlation between the rate of formation of aggregates and the concentration of the protein [7].
Another factor that affects aggregation is presence of additional compounds, which are usually small molecules that are aromatic in nature [7]. It was reported that addition of certain small molecule modulators to the amyloid proteins before the beginning of aggregation promotes nontoxic amyloid-beta aggregate formation and reduces the toxic formation of amyloid-beta aggregates [14]. Also, certain small molecules are nontoxic to humans and are blood-brain barrier permeable; as a result, these molecules have potential to be used for therapeutic application since they are not harmful to humans and can change the outcome of aggregation process [8]. Figure 2 shows an example of how the small molecules can change the formation of the aggregates when added to the protein solution. Figure 2. Untreated sample of the protein results in aggregation and formation of amyloid fibrils. Treatment of the protein solution with the small molecules has the following outcomes: addition of Brilliant Blue G (BBG) resulted in formation of fragmented amyloid fibrils; addition of methylene blue (MB) resulted in formation of distorted amorphous aggregate; and addition of Erythrosine B (ER) yielded formation of protofibril aggregates. [8].
As follows from figure 2, all three of the tested molecules are biocompatible and have potential to be used to remove insoluble amyloid fibrils deposited in the human brain [8]. Furthermore, these molecules are promising candidates to be used to control the process of aggregation and change the morphology and properties of the final aggregates.
Techniques Used to Study Aggregation
Aggregation is one of the processes that is challenging to monitor. First of all, it is difficult to keep track of all the amino acid residues because proteins have very long polypeptide chains. Moreover, when data can be quiet complicated unless one knows what to look for and when [9]. Fortunately, the use of shorter peptides allowed for more possibilities in the analysis of aggregates and provided a baseline point of reference for the aggregated structures.
One of the most obvious methods by study the aggregates is by the use of microscopy, both Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) can be used to take images of the aggregated peptides and later to measure and compare the dimensions of the fibrils [8]. Furthermore, X-ray diffraction can also be used to measure the parameters of the formed structures [6].
Another simple technique is dye staining with Congo red. This Thioflavin T Assay is useful because amyloid formation can by observed after excitation at 450 nm produces fluorescence signal at 482 nm [6].
Additional technique that remains among the most commonly used to study aggregation is circular dichroism (CD). CD refers to the differential absorption of right- and left- circularly polarizes light. The usual method is based on the fingerprinting of features in the far-UV region [6]. CD spectra is used to distinguish among the various structures of the aggregates, whether they are alpha helices, beta sheets, or random coil. Furthermore, Vibrational Circular Dichroism (VCD) is a classical and still one of the most useful tools for exploring the conformations of proteins. Amide I regions of the β-sheet aggregates are very structurally sensitive due to the influence of the hydrogen bonding and strong vibrational mixing between the amide I vibrations of adjacent peptide groups. This makes it very simple to monitor the VCD band signal for amide I while monitoring the progress of aggregation process. In addition, the VCD provides enough data to distinguish among the stacked, parallel, antiparallel, twisted, bent β-sheets [19].
IR spectroscopy is well used for studying β-sheet aggregates because they give distinctive amide I bands in the IR spectrum. The change in the intensity of the peaks for the amide I region can be used to monitor the progress of the aggregation process while continuously collecting of the data [18]. In addition, inclusion of specific isotope labels in the peptide gives residue-level structural details that can be studied while aggregation occurs [15].
Raman Spectroscopy is another technique that is used by many chemists to monitor the progress of aggregation [16-17]. The advantage of UV-resonance Raman spectroscopy is that it reduces the complexity of peptide and protein by selective enhancement of a few modes. Moreover, it has certain advances for structural analysis. First of all, measurements can be done at millimolar concentration. Secondly, more structure-sensitive bands appear with sufficient intensity in Raman than in the IR spectra. Finally, interferences with signals from amino acid residues are avoided [17].
Finally, a lot of progress has been achieved in the theoretical field. For example, density functional theory has been successfully used to calculate the strengths of the hydrogen bonds between the water solvate molecules and the protein backbone in a polyproline II conformation, which is usually used as a control for aggregation reactions. As a result of the calculations it was concluded that as the hydration shell of the protein increases, the strength of the hydrogen bonds weakens significantly [20]. That means that depending on the amount of water present in the sample during aggregation, the process may have different outcome.
Conclusion
Although aggregation, and amyloid formation in particular, has been verified to have an association with multiple neurodegenerative diseases, the mechanism of this process is still not fully understood among the scientific community. Nonetheless, a lot of work has been done in the attempts to identify key factors that promote as well as inhibit aggregation of peptides. In addition, a number of various techniques have been identified to be extremely useful in monitoring the progress of aggregation and analyzing the final products of the process. Hopefully in not so distant future we will be able to understand the mechanisms by which aggregation occurs well enough to decrease the affects of neurodegenerative diseases and maybe even cure them.
2. Checler, F. (2011). Journal of Neurochemistry special issue on Alzheimer’s disease: ‘amyloid cascade hypothesis – 20 years on’. Journal of Neurochemisty. http://dx.doi.org/10.1111/j.1471-4159.2011.07603.x
3. Brodsky, B. (2012). A Peptide Study of the Relationship Between the Collagen Triple-Helix and Amyloid. Biopolymers http://dx.doi.org/10.1002/bip.22070
4. Spillantini, M. (1998). Lewy body diseases and multiple system atryphy as α-Synucleinopathies. Molecular Psychiatry. http://dx.doi.org/10.1038/sj.mp.4000458
5. Dobson, C. (2003). Protein Folding and Disease: a view from the first Horizon Symposium. Nature Reviews Drug Discovery. http://dx.doi.org/10.1038/nrd1013
7. Jones, O. (2012) Inhibiting, Promoting, and Preserving Stability of Functional Protein Fibrils. Soft Matter. http://dx.doi.org/10.1039/c1sm06643a
8. Irwin, J. (2012). Different Fates of Alzheimer’s Disease Amyloid-β Fibrils Remodeled by Biocompatible Small Molecules. Bio Macromolecules. http://dx.doi.org/10.1021/bm3016994
11. Manno, M. (2010). Amyloid Gels: Precocious Appearance of Elastic Properties during the Formation of an Insulin Fibrillar Network. Landmuir Letter. http://dx.doi.org/10.1021/la903340v
12. Arora, A. (2004). Insulin amyloid fibrillation at above 100C:New insights into protein folding under extreme temperatures. Protein Science. http://dx.doi.org /10.1110/ps.04823504
13. Torres, J. (2012). The Folding of Acetyl(Ala)28NH2 and Acetyl(Ala)40NH2 Extended Strand Peptides into Antiparallel β-Sheets. A Density Functional Theory Study of β-Sheets with β-Turns. The Journal of Physical Chemistry http://dx.doi.org/10.1021/jp3094947
14. Benilova, I. (2012). The toxic Aβ oligomer and Alzheimer's disease: an emperor in need of clothes. Nature Neuroscience. http://dx.doi.org/10.1038/nn.3028
16. Nunsing, W. (2012). Vibrational spectroscopy of self-assembling aromatic peptide derivates. Journal of Raman Spectroscopy. http://dx.doi.org/10.1002/jrs.4063
17. Schweitzer-Stenner, R. (2001). Visible and UV-resonance Raman spectroscopy of model peptides. Journal of Raman Spectroscopy. http://dx.doi.org /10.1002/jrs.757
18. Decatur, S. (2005). Experimental Evidence for the Reorganization ofβ-Strands within Aggregatesof the Aβ(16-22) Peptide. JACS http://dx.doi.org/10.1021/ja054663y
19. Schweitzer-Stenner, R. (2012). Simulated IR, Isotropic and Anisotropic Raman, and Vibrational Circular Dichroism Amide I Band Profiles of Stacked β -Sheets J. Phys. Chem. http://dx.doi.org/10.1021/jp2112445
20. Ireta, J. (2012). Microsolvation Effects on the Stability of Polyalanine in Extended and Polyproline II Conformation. International Journal of Quantum Chemistry. http://dx.doi.org/10.1002/qua.24246
Introduction
Amino acids are like building blocks of human body. They are connected in a manner that is dictated by the DNA and they form proteins, which are responsible for most of the processes that happen in human bodies. One of the defining characteristics of living organisms is the ability of its smallest molecular structures to self-assemble with great precision. The folding of the proteins in human bodies are one of the most fundamental and universal examples of such self-assembly processes; nonetheless, the mechanisms by which these processes work is still under investigation by scientists in multiple fields [1].
Cells have certain quality control mechanisms that ensure that the proteins are folded correctly because failure to do so gives rise to malfunctions of normal processes and hence to disease. Among all the possible structures that can be formed by the polypeptide chains, amyloid fibril aggregates are considered to be of one of the most biologically important protein structures due to their implication in numerous neurodegenerative diseases [2]. The diseases that have been connected to amyloid fibril formation include Alzheimer’s disease [3], Parkinson’s disease, and prion disease [4]. As a result, understand how and why aggregation of peptide chains happens is crucial in the fight against these diseases.
Fibrillization
Based on the conditions and the environment, protein can take on multiple conformations. Figure 1 shows some of the possible structures that can be formed from the polypeptide chains.
Figure 1. A unified view of some of the types of structures that can be formed by the polypeptide chains [2].
The fibrillization of peptides, as mentioned earlier, is relevant to many diseases and is based on the deposition of amyloids. The term amyloid refers to protein deposits that adopt fibrils formation based on the cross-β sheet structure [6]. Structuring through β-sheets is common in many protein aggregates. During protein fibril formation, β-sheets must aggregate in a rigid fashion to create the specific structure that is characteristic of the amyloids [7].
One of the major proposed theories in development of amyloid diseases is the amyloid cascade hypothesis, which states that malfunction of proteins leads to the production of amyloid proteins and eventually aggregation into cell-disrupting amyloid plaque [2]. In addition, it was verified that amyloid fibrils could be transmitted to other sites to promote further fibril formation. At this point the disease becomes infectious [7]. This fact makes it crucial to figure out how to control the formation of fibrils.
Many experiments that were done during research of the fibril formation show that there are certain factors that either promote or inhibit the formation of the fibrils. Over time it was been verified that the following factors are important in the formation of amyloid fibrils: concentration, ionic strength, pH level, solid substrates, temperature [7]. Although more research is being conducted to figure out how exactly these factors affect the onset of aggregation, they all have significant role in assembly of the fibrils.
While it has been verified that in order for aggregation to start and amyloid fibrils to form, certain conditions mush be met; once amyloid fibrils do form, they are very thermodynamically stable and the reverse process does not occur spontaneously [8]. Therefore, scientists are trying to find strategies to either inhibit aggregation altogether and prevent the amyloid fibrils from forming or to change the way aggregation occurs.
Factors that Affect Aggregation
Fortunately, the topic of aggregation of peptides had been fairly popular and a lot of progress is being made in discovering how to control the outcomes of the processes. More research became available because many scientists started using short peptides as models for amyloid formation. Since most proteins have long polypeptide chains, it makes them a little more complicated to work with. Moreover, the synthesis of the large peptides is expensive and difficult. As a result, much current work is done on short peptide sequences [9].
Since work with shorter peptide sequences allowed for more detailed research, there are several, both physiological and nonphysiological, conditions that have been investigated to break, disrupt, and destabilize amyloid fibrils. First of all, actual physical energy has been applied to break down fibril aggregates. There have been experiments conducted where ultrasonification treatments of the aggregated peptides resulted in shorted amyloid fibril fragments. In addition to breaking the fibrils into smaller fragments, ultrasonification is a powerful technique, which allows for producing monodispersed fibrils with a well-defined molecular size [10]. Also, the opposite of this concept is useful in cases where undisturbed aggregation is used to produce elongated aggregates during the lag phase, or the time before aggregation is complete [11].
Another factor that affects the process of aggregation is high temperature. It has been found during one study that temperatures above 100 °C happen to disrupt the strength of hydrogen-bond networks [12] that are crucial for the rigid amyloid fibril structure [13]. In the observed experiments no amyloid formation was observed at and over 140 °C. In addition, the morphology of actual fibrils changed with increasing temperature. It was found that as the temperature increases, the size of the fibrils decreases and their shape is not as linear as the shape of the fibrils at lower temperatures [12].
Additionally, charge is very important factor in the process of aggregation. Charge is crucial for the structure stability; therefore drastic changes in the pH level or the level of salt in the environment can lead to fibril destruction [8]. Through changing the charge of the amino acids along the peptide, the pH value determines intramolecular protein interactions and electrostatic intermolecular interactions between proteins. As pH level decrease, the protein becomes more acidic and the non-selective aggregation is favored. Experimentally this results in the formation of stiff fibril morphology, although β-sheet alignment may be found in aggregates at all pH levels [7].
Factors like protein concentration and solvent quality also affect aggregation. Both of them have strong influence on what type of aggregates will form, for example fibrils or amorphous assemblies. In combination with the temperature, these two factors also affect how fast the aggregation is going to proceed. In higher temperature there is clear correlation between the rate of formation of aggregates and the concentration of the protein [7].
Another factor that affects aggregation is presence of additional compounds, which are usually small molecules that are aromatic in nature [7]. It was reported that addition of certain small molecule modulators to the amyloid proteins before the beginning of aggregation promotes nontoxic amyloid-beta aggregate formation and reduces the toxic formation of amyloid-beta aggregates [14]. Also, certain small molecules are nontoxic to humans and are blood-brain barrier permeable; as a result, these molecules have potential to be used for therapeutic application since they are not harmful to humans and can change the outcome of aggregation process [8]. Figure 2 shows an example of how the small molecules can change the formation of the aggregates when added to the protein solution.
Figure 2. Untreated sample of the protein results in aggregation and formation of amyloid fibrils. Treatment of the protein solution with the small molecules has the following outcomes: addition of Brilliant Blue G (BBG) resulted in formation of fragmented amyloid fibrils; addition of methylene blue (MB) resulted in formation of distorted amorphous aggregate; and addition of Erythrosine B (ER) yielded formation of protofibril aggregates. [8].
As follows from figure 2, all three of the tested molecules are biocompatible and have potential to be used to remove insoluble amyloid fibrils deposited in the human brain [8]. Furthermore, these molecules are promising candidates to be used to control the process of aggregation and change the morphology and properties of the final aggregates.
Techniques Used to Study Aggregation
Aggregation is one of the processes that is challenging to monitor. First of all, it is difficult to keep track of all the amino acid residues because proteins have very long polypeptide chains. Moreover, when data can be quiet complicated unless one knows what to look for and when [9]. Fortunately, the use of shorter peptides allowed for more possibilities in the analysis of aggregates and provided a baseline point of reference for the aggregated structures.
One of the most obvious methods by study the aggregates is by the use of microscopy, both Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) can be used to take images of the aggregated peptides and later to measure and compare the dimensions of the fibrils [8]. Furthermore, X-ray diffraction can also be used to measure the parameters of the formed structures [6].
Another simple technique is dye staining with Congo red. This Thioflavin T Assay is useful because amyloid formation can by observed after excitation at 450 nm produces fluorescence signal at 482 nm [6].
Additional technique that remains among the most commonly used to study aggregation is circular dichroism (CD). CD refers to the differential absorption of right- and left- circularly polarizes light. The usual method is based on the fingerprinting of features in the far-UV region [6]. CD spectra is used to distinguish among the various structures of the aggregates, whether they are alpha helices, beta sheets, or random coil. Furthermore, Vibrational Circular Dichroism (VCD) is a classical and still one of the most useful tools for exploring the conformations of proteins. Amide I regions of the β-sheet aggregates are very structurally sensitive due to the influence of the hydrogen bonding and strong vibrational mixing between the amide I vibrations of adjacent peptide groups. This makes it very simple to monitor the VCD band signal for amide I while monitoring the progress of aggregation process. In addition, the VCD provides enough data to distinguish among the stacked, parallel, antiparallel, twisted, bent β-sheets [19].
IR spectroscopy is well used for studying β-sheet aggregates because they give distinctive amide I bands in the IR spectrum. The change in the intensity of the peaks for the amide I region can be used to monitor the progress of the aggregation process while continuously collecting of the data [18]. In addition, inclusion of specific isotope labels in the peptide gives residue-level structural details that can be studied while aggregation occurs [15].
Raman Spectroscopy is another technique that is used by many chemists to monitor the progress of aggregation [16-17]. The advantage of UV-resonance Raman spectroscopy is that it reduces the complexity of peptide and protein by selective enhancement of a few modes. Moreover, it has certain advances for structural analysis. First of all, measurements can be done at millimolar concentration. Secondly, more structure-sensitive bands appear with sufficient intensity in Raman than in the IR spectra. Finally, interferences with signals from amino acid residues are avoided [17].
Finally, a lot of progress has been achieved in the theoretical field. For example, density functional theory has been successfully used to calculate the strengths of the hydrogen bonds between the water solvate molecules and the protein backbone in a polyproline II conformation, which is usually used as a control for aggregation reactions. As a result of the calculations it was concluded that as the hydration shell of the protein increases, the strength of the hydrogen bonds weakens significantly [20]. That means that depending on the amount of water present in the sample during aggregation, the process may have different outcome.
Conclusion
Although aggregation, and amyloid formation in particular, has been verified to have an association with multiple neurodegenerative diseases, the mechanism of this process is still not fully understood among the scientific community. Nonetheless, a lot of work has been done in the attempts to identify key factors that promote as well as inhibit aggregation of peptides. In addition, a number of various techniques have been identified to be extremely useful in monitoring the progress of aggregation and analyzing the final products of the process. Hopefully in not so distant future we will be able to understand the mechanisms by which aggregation occurs well enough to decrease the affects of neurodegenerative diseases and maybe even cure them.
References:
1. Dobson, C. (2003). Protein Folding and Misfolding. Nature
http://dx.doi.org/10.1038/nature02261
2. Checler, F. (2011). Journal of Neurochemistry special issue on Alzheimer’s disease: ‘amyloid cascade hypothesis – 20 years on’. Journal of Neurochemisty.
http://dx.doi.org/10.1111/j.1471-4159.2011.07603.x
3. Brodsky, B. (2012). A Peptide Study of the Relationship Between the Collagen Triple-Helix and Amyloid. Biopolymers
http://dx.doi.org/10.1002/bip.22070
4. Spillantini, M. (1998). Lewy body diseases and multiple system atryphy as α-Synucleinopathies. Molecular Psychiatry.
http://dx.doi.org/10.1038/sj.mp.4000458
5. Dobson, C. (2003). Protein Folding and Disease: a view from the first Horizon Symposium. Nature Reviews Drug Discovery.
http://dx.doi.org/10.1038/nrd1013
6. Hamley, I. (2007). Peptide Fibrillization. Angew. Chem. Int. Ed.
http://dx.doi.org/10.1002/anie.200700861
7. Jones, O. (2012) Inhibiting, Promoting, and Preserving Stability of Functional Protein Fibrils. Soft Matter.
http://dx.doi.org/10.1039/c1sm06643a
8. Irwin, J. (2012). Different Fates of Alzheimer’s Disease Amyloid-β Fibrils Remodeled by Biocompatible Small Molecules. Bio Macromolecules. http://dx.doi.org/10.1021/bm3016994
9. Gazit, E. (2005). Mechanisms of amyloid fibril self-assembly and inhibition. The FEBS Journal.
http://dx.doi.org/10.1111/j.1742-4658.2005.05022.x
10. Chatani, E. (2009). Ultrasonication-dependent production and breakdown lead to minimum-sized amyloid fibrils. PNAS.
http://dx.doi.org/10.1073/pnas.0901422106
11. Manno, M. (2010). Amyloid Gels: Precocious Appearance of Elastic Properties during the Formation of an Insulin Fibrillar Network. Landmuir Letter.
http://dx.doi.org/10.1021/la903340v
12. Arora, A. (2004). Insulin amyloid fibrillation at above 100C:New insights into protein folding under extreme temperatures. Protein Science.
http://dx.doi.org /10.1110/ps.04823504
13. Torres, J. (2012). The Folding of Acetyl(Ala)28NH2 and Acetyl(Ala)40NH2 Extended Strand Peptides into Antiparallel β-Sheets. A Density Functional Theory Study of β-Sheets with β-Turns. The Journal of Physical Chemistry
http://dx.doi.org/10.1021/jp3094947
14. Benilova, I. (2012). The toxic Aβ oligomer and Alzheimer's disease: an emperor in need of clothes. Nature Neuroscience.
http://dx.doi.org/10.1038/nn.3028
15. Decatur, S. (2005). Intersheet rearrangement of polypeptides during nucleation of -sheet aggregates. PNAS
http://dx.doi.org/10.1073/pnas.0502804102
16. Nunsing, W. (2012). Vibrational spectroscopy of self-assembling aromatic peptide derivates. Journal of Raman Spectroscopy.
http://dx.doi.org/10.1002/jrs.4063
17. Schweitzer-Stenner, R. (2001). Visible and UV-resonance Raman spectroscopy of model peptides. Journal of Raman Spectroscopy.
http://dx.doi.org /10.1002/jrs.757
18. Decatur, S. (2005). Experimental Evidence for the Reorganization ofβ-Strands within Aggregatesof the Aβ(16-22) Peptide. JACS
http://dx.doi.org/10.1021/ja054663y
19. Schweitzer-Stenner, R. (2012). Simulated IR, Isotropic and Anisotropic Raman, and Vibrational Circular Dichroism Amide I Band Profiles of Stacked β -Sheets J. Phys. Chem.
http://dx.doi.org/10.1021/jp2112445
20. Ireta, J. (2012). Microsolvation Effects on the Stability of Polyalanine in Extended and Polyproline II Conformation. International Journal of Quantum Chemistry.
http://dx.doi.org/10.1002/qua.24246